: Most of what is known about the neural basis of perception comes from detailed correlation and hypothesis testing of putative neural mechanisms against detailed psychophysical outcomes. Although most of those studies have employed peripheral neural data the hypothesis testing methods used in those studies applies equally to the cortical mechanisms underlying perception. The first ai is to identify which cortical neuronal populations and neural coding mechanism can and which cannot account for the primate capacity for tactile form and texture perception. This will be attacked in three ways. The first employs exactly the same stimuli in cortical neurophysiological experiments as in psychophysical experiments. The question in areas 3b, 1 and 2 and the areas in and around Sll cortex will be whether the cortical neural responses to these stimuli can account for the associated psychophysics. This correlative strateg depends on the cross-species assumption that psychophysical capacity and neura mechanisms in man and monkey are identical. To test this assumption, new psychophysical studies will be undertaken in both man and monkey involving the discrimination of sinusoidal gratings. The fourth approach is different and is based on the substantial evidence that SAI afferents are responsible for form and texture perception while RA afferents are responsible for the detection of motion. By selective activation of SAI and RA afferents, we hope to obtain additional clues to the central pathways subserving these functions. Identifying the neurons responsible for form and texture processing is only part of the story, the second part is how they do it. Two developments during the current funding period allow us to attack this question more effectively than in the past. One is a new tactile stimulator with 400 independently controllable probes. The second is the development of new stimulus methods and analytical techniques for investigating the spatial distribution of excitation and inhibition that determines a neuron's response properties. These have allowed us to determine the receptive field structure of 247 neurons in area 3 of the alert monkey. For the first time, we have a picture of the receptive field organization in area 3b. The same methods, made more powerful with the new stimulator, will be used to study neuronal responses in the remainder of S and in Sll cortex.